The present invention relates to electrostatic particle precipitators which are capable of removing with a high collecting efficiency dust particles that are difficult to collect.
The flue gas discharged from all types of boilers for business and industrial purposes, cement calcining furnaces and metallurgic sintering furnaces involve a great amount of dust particles. To remove these particles fabric filters, cyclones or electrostatic particle precipitators are being used. However, as regulations on the allowable number of dust particles in the flue gas are becoming stricter and as attempts are made for attaining such regulated allowable values without fail with the least energy and over an extended period of time, it is almost impossible to accomplish the purpose with conventional devices. For example, the dust particles contained in the flue gas from coal-burning boilers in the order of several scores g/m.sup.3 N can be reduced to somewhere around several thousand mg/m.sup.3 N by channelling the flue gas through a cyclone and can be further reduced to around 50-100 mg/m.sup.3 N by treating it with an electrostatic particle precipitator. However, for reducing the contained dust particles further, e.g., to less than 20-30 mg/m.sup.3 N, an injection of ammonia gas or spraying of water has been used, but with a success only in laboratory. There has been no instance of industrial success where economics, prevention of secondary pollution and the safety in operation are taken into consideration.
In order to improve the collecting efficiency of the electric particle precipitator, it has been suggested that a device be used to remove the dust particles accumulated on the electrodes by moving the collecting electrodes to a region which is free from the impact of the flue gas stream. See, for example, U.S. Pat. No. 3,912,467 (Oct. 4, 1975).
FIG. 1 of the present application shows the collecting electrode moving system used in the electrostatic particle precipitator disclosed in said U.S. Pat. No. 3,912,467. In the drawing, 1 is the collecting electrode moving system arranged in a plural number in parallel runs. On the main driving wheels 2 and the subject driving wheels 3, endless chains 5 are disposed. Only the wheels and the chain on the left side of the device are shown in FIG. 1. A plurality of the collecting plate electrodes 4 are arranged on the chains. They are vertically rectangular in their shape and are mounted in series with a fixed distance between each other. Discharge electrodes 6 are positioned in between each of the arrays of moving collecting plate electrodes. The treating flue gas flows from the left to right (as viewed in FIG. 1) toward the rear of the system.
In this structure, by applying a high voltage between the discharge electrodes 6 and the collecting plate electrodes 4, the space in which the discharge electrodes 6 are located forms a dust collecting region and the dust particles contained in the flue gas are collected on the surface of the collecting plate electrodes 4. The lower space where no discharge electrodes 6 exists is the region where no dust is collected on the electrodes 4. In this region the dust particles accumulated on the surface of the electrodes 4 are scraped off by a removing device 7.
In practice, this type of electrostatic particle precipitator comprises a long track formed by a pair of endless chains 5 running around a plurality of bends as dictated by the number of wheels 3. This results in a complex structure and one which is inconvenient in that its overall dimension is large as compared with the fixed type particle precipitator. In addition, generally the ingredients of the dust particles contained in the flue gas discharged from such furnaces as boilers involve silica and alumina which make the structural steel materials vulnerable to wear. Also, the conventional electrode moving type electrostatic particle precipitator as described above requires that a large tension be applied to the endless chains 5. This is so because of the long length of the chains 5 and the number of wheels 3. Since the wear-rendering dust particles become attached to these chains, they and the subject driving wheels 3 are particularly subject to excessive wear damage. This wear can occur in a short period of time. It is therefore difficult to maintain a stabilized dust collecting operation over a long period of time in situations involving wear-rendering dust particles.